Do they allow 15-year-olds to have sex with 18-year-olds?
A two-year age difference isn’t particularly alarming, and dating is fairly standard at that age.
Did he mysteriously go from child to adult in one day? That’s right - your daughter is not a child anymore, no matter how much you want her to be. Unless this guy is into drugs and alcohol, or is a juvenile delinquent, you have no cause for concern.
Did something magically happen when he crossed that birthday threshold? If they do, then calm down and let them have whatever relationship they want.
16 and 18 are safe in most States - unless you’re California, where apparently no one is safe until they’re 18.
And no one has to actually report this for them to get caught and prosecution to occur. If their relationship is no longer legal where you live, and you report this guy, one outcome I can pretty much guarantee for you - your daughter will never, ever trust you again. You will basically end your relationship with her for good.
It's because of this urban legend that has gone around forever.